Bonjour Sue !
I completely how our formulation was confusing here.
What I meant was "avoir du mal à" never mean "to struggle" in the sense of "fight".
Thanks to you, I've now clarified this sentence and added a counter example as well.
As for the sentence you mentioned, you would indeed say:
"J'ai du mal à monter la colline."
Merci et à bientôt !